The pathways and mechanisms of transepithelial transport of NaCl are being investigated in the proximal tubule of the Necturus kidney. Sodium and potassium sensitive intracellular microelectrodes were constructed to monitor intracellular sodium and potassium activity and to determine cellular responsiveness to changes in extracellular sodium or chloride concentration. Tracer chloride fluxes across the tubule were als measured to quantitate the chloride movements across the tubule cells and through the shunt pathway.